Education and Communications

Why Doesn’t The Leaning Tower Of Pisa Fall Over? – Alex Gendler

In 1990, the Italian government
enlisted top engineers to stabilize Pisa’s famous Leaning Tower. There’d been many attempts to right the
tower during its 800 year history, but this team’s computer models revealed
the urgency of their situation. They projected the tower would topple if
it reached an angle of 5.44 degrees— and it was currently leaning at 5.5. No one knew how the tower was still
standing, but the crisis was clear: they had to solve a problem that
stumped centuries of engineers, and they needed to do it fast. To understand their situation, it’s helpful to understand why the
tower tilted in the first place. In the 12th century, the wealthy
maritime republic of Pisa set about turning its cathedral square
into a magnificent landmark. Workers embellished and enlarged
the existing church, and added a massive domed
baptistry to the plaza. In 1173, construction began on a
free-standing campanile, or bell tower. The engineers and architects of the
time were masters of their craft. But for all their engineering knowledge, they knew far less about the
ground they stood on. Pisa’s name comes from a
Greek word for “marshy land,” which perfectly describes the clay, mud,
and wet sand below the city’s surface. Ancient Romans counteracted similar
conditions with massive stone pillars called piles which rest on
Earth’s stable bedrock. However, the tower’s architects believed
a three-meter foundation would suffice for their relatively short structure. Unfortunately for them,
less than five years later, the tower’s southern side
was already underground. Such a shifting foundation would
normally have been a fatal flaw. If workers added more weight, the pressure from upper stories
would sink the structure and fatally increase the lean. But construction halted at the
fourth story for nearly a century as Pisa descended into prolonged warfare. This long pause allowed
the soil to settle, and when construction
began again in 1272, the foundation was on
slightly more stable footing. Under the direction of
architect Giovanni di Simone, workers compensated for
the tower’s minor tilt by making the next few floors taller
on the southern side. But the weight of the extra masonry
made that side sink even deeper. By the time they completed the seventh
floor and bell chamber, the angle of the tilt was 1.6 degrees. For centuries, engineers tried numerous
strategies to address the lean. In 1838, they dug a walkway around the
base to examine the sunken foundation. But removing the supporting sand only
worsened the tilt. In 1935, the Italian Corps of Engineers
injected mortar to strengthen the base. However, the mortar wasn’t evenly
distributed throughout the foundation, resulting in another sudden drop. All these failed attempts, along with
the ever-sinking foundation, moved the tower closer to
its tipping point. And without definitive knowledge
of the soil composition, engineers couldn’t pinpoint
the tower’s fatal angle or devise a way to stop its fall. In the years following WWII, researchers developed tests to
identify those missing variables. And in the 1970’s, engineers calculated
the curved tower’s center of gravity. With this data and new
computing technology, engineers could model how stiff
the soil was, the tower’s trajectory, and the exact amount of excavation
needed for the tower to remain standing. In 1992, the team drilled diagonal tunnels to remove 38 cubic meters of soil
from under the tower’s north end. Then, they temporarily counterbalanced
the structure with 600 tons of lead ingots before anchoring the base
with steel cables. More than six centuries
after its construction, the tower was finally straightened…
to a tilt of about four degrees. No one wanted the tower to fall, but they also didn’t want to lose the
landmark’s most famous feature. Today the tower stands at
55– or 56– meters tall, and it should remain stable
for at least 300 years as a monument to the
beauty of imperfection.
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